This invention relates generally to an improved dispenser of the reciprocating piston type for discharging liquids from a portable container, and more particularly to such dispenser as having a feature which prevents any hydraulic lock which would interfere with a full depression of the discharge head into a lock-down position, as well as a feature which facilitates easy unlocking of a seal lock between the head and a retaining collar on the container closure.
Liquid dispensers generally of the type described herein, as including a stationary pump cylinder for direct association with a container and a reciprocable piston unit which is manipulated to effect discharge of the container contents, are known as having means provided for temporarily locking the piston against relative displacement in a position such that the dispenser is completely sealed, thereby preventing leakage from the container during shipping or storage, even though the container be inverted. U.S. Pat. Nos. 3,084,873, 3,248,021 and 3,237,571 disclose liquid dispensers having plunger lock-down means for immobilizing and retaining the plunger in a depressed position. Such means is generally in the form of co-acting threads provided on the plunger head and a collar portion of the container cap. Thus, the plunger is locked down into its depressed position by screwing down the head within the collar. And, co-acting sealing surfaces are made to interengage when the plunger is fully depressed and locked so as to effect a seal against leakage of liquid outwardly of the container from the space between the plunger and the pump cylinder. While these plunger lock-down and sealing mechanisms have generally performed satisfactorily, they are not without their shortcomings. For example, the interengaging sealing surfaces must be provided in addition to the co-acting screw threads for respectively producing a seal and a lock which require additional time and materials to achieve these intended functions. Moreover, if the container cap is threaded down on to the container neck, as in the U.S. Pat. Nos. 3,084,873 and 3,237,571 dispensers, unthreading the plunger head relative to the collar for unlocking the plunger oftentimes results in an inadvertent unthreading and loosening of the cap from the container.
In lieu of co-acting locking threads, the plunger has been maintained in a downwardly depressed immobilized position by means of a protective overcap (as in U.S. Pat. No. 2,956,509) which snaps down over the collar but which has generally proven cumbersome because of the need for such an overcap.
And, means are provided in U.S. Pat. Nos. 3,248,021 and 3,237,571 for preventing flow of liquid, when in the plunger lock-down position, past the inlet valve and outwardly through the discharge passage, or between the pump cylinder and the plunger, when the dispenser is inverted. In the former, the inlet ball valve is spring biased into a fully seated position and a lip seal on the plunger is engaged in the lock-down position, and in the latter, the lower end of an inlet valve is seated below the valve openings in the lock-down position of the plunger. In any event, with or without such anti-leakage means, the liquid dispensers of the aforedescribed types are oftentimes difficult to lock down during shipment and storage due to a hydraulic lock effect which arises especially after the pump has been primed with liquid to be dispensed. This condition renders it difficult if not impossible to fully depress the plunger into its immobilized and locked position because of the resistance from the pressurized liquid which remains in the pump cylinder.